Gas generators used in safety devices for motor vehicles usually contain a propellant based on sodium azide. Sodium azide is toxic, however, as a result of which there have been numerous attempts to develop azide-free propellants.
The German Offenlegungsschrift DE 195 05 568 A1 discloses azide-free propellants of this type which contain at least one compound from the group of tetrazoles, triazoles, triazines, cyanic acid, urea, derivatives or salts thereof as fuel, compounds from the group of peroxides, nitrates, chlorates or perchlorates as oxidation agents, and in addition burn-off moderators which are suitable for influencing the burn-off and the rate thereof by heterogeneous or homogeneous catalysis, as well as optionally additional charges which are suitable for reducing the proportion of toxic gases which are produced during the combustion of the propellant. Propellants are preferred which contain biuret, guanidine, nitroguanidine, guanidine nitrate, aminoguanidine, aminoguanidine nitrate, thiourea, triaminoguanidine nitrate, aminoguanidine hydrogencarbonate, azodicarboxylic acid diamide, dicyandiainide dinitrate, dicyandiamidine sulphate, tetrazene and/or semicarbazide nitrate as well as urethanes, ureides and derivatives thereof as fuel, at least two compounds from the group of peroxides, nitrates, chlorates or perchlorates as oxidation agents, as well as zinc oxide or carbonates of zinc or calcium as burn-off moderators.
A disadvantage of the propellants described in DE 195 05 568 A1 is the use of nitrates as oxidation agents, since under unfavourable circumstances relatively large quantities of nitrous gases NO.sub.X can be formed during the thermal decomposition of these nitrates. For this reason it is necessary to use aggregates in order to reduce the proportion of NO.sub.X in the gas mixture produced. A further disadvantage of the propellant mixtures described lies in the high burn-off temperatures and explosion heat caused by these mixtures. Tests with various mixtures of propellants have shown, however, that high burn-off temperatures always result in high concentrations of harmful gases in the gas mixture produced.
U.S. Pat. No. 5,467,715 discloses a gas-producing composition with between substantially 20 and 40% by weight of a fuel, between substantially 20 and 80% by weight of an oxidant, and the remainder customary additives. From 50 to substantially 85% by weight of the propellant consists of a triazole or tetrazole, between substantially 15 and substantially 50% by weight of the propellant consists of a water-soluble fuel, such as for example guanidine nitrate, ethylenediamide dinitrate or similar compounds. At least substantially 20% by weight of the oxidant contains a transition-metal oxide, and the remainder of the oxidant is formed from alkali or alkaline-earth metal nitrates, chlorates or perchlorates. The use of transition-metal oxides as the main constituent of the oxidant is intended to lead to a lowering in the combustion temperatures and to a reduction in the proportion of toxic gases. The production of the compositions described is carried out in a wet-chemical manner by mixing together the constituents of the propellant mixture in an aqueous slurry.
The 5-aminotetrazole preferably used as a fuel in accordance with U.S. Pat. No. 5,467,715 is extremely hygroscopic, however, and thus makes exacting demands upon environmental conditions both during storage and in the processing of the propellant mixtures. Because of the high degree of hygroscopicity of these propellant mixtures, it is additionally necessary to ensure the sealing tightness of the gas generator containing this propellant over the entire duration of its use, since absorption of water has a very powerful effect upon the burn-off characteristics of the fuel. Water in the form of water of crystallization is bonded by 5-aminotetrazole and is released by it again only under extreme conditions.
A further disadvantage is the treatment of the described propellant mixture in a wet process. This wet process requires additional costly steps of moistening and drying, which preclude an economical production of the propellant. In addition, it has been found in tests that 5-aminotetrazole and copper oxide (CuO) are incompatible with each other. Ballistic tests after long-term temperature storage at 110.degree. C. over 400 hours have revealed greatly increased combustion-chamber pressures in the case of a propellant mixture of this type. There is therefore no guarantee of the long-term stability of these propellant mixtures.